The present invention relates to a flowmeter for measuring the quantity of flow of a fluid such as gas and, more particularly, to a flowmeter capable of accurately measuring a quantity of flow over a wide range of the quantity of flow.
1. Background Art
Some flowmeters for measuring the quantity of flow of a fluid, such as gas, calculate the quantity of flow by disposing a flow velocity sensor in a flow path and multiplying the flow velocity obtained by the flow velocity sensor by the cross-sectional area of the flow path.
FIG. 29 shows a specific configuration of such a conventional flowmeter. In the flowmeter, a flow velocity sensor 1 is disposed in the center of a fluid flow path in a pipe 2. A flow quantity calculating unit 3 calculates the quantity of flow by multiplying the flow velocity in the center of the flow path obtained by the flow velocity sensor 1 by the cross-sectional area of the pipe 2 and the quantity of flow is displayed on a display unit 4. In order to maintain the high accuracy in flow quantity measurement by the flow velocity sensor, the flow velocity sensor 1 has to be disposed in the most stable flow of the fluid.
The conventional flowmeter has, however, a problem such that a mounting position of the flow velocity sensor 1 is difficult to be determined since a drift occurs in the pipe 2 depending on the quantity of flow. It is necessary to set the flow velocity sensor 1 in a limited range of flow quantity where the drift is small. The measurable range of the flow quantity is accordingly narrowed and it is difficult to accurately measure the flow quantity of gas over a wide flow quantity range. The drift denotes here that the flow velocity varies according to places. The flow quantity range denotes the range of the quantity of flow. The definition will be the same in the following description.
As described above, in the conventional flowmeter, it is difficult to set a sufficiently wide flow quantity measurement range by using a single flow velocity sensor. In order to solve the problem, the following method is considered. The flow quantity measurement range is divided into, for example, two ranges of a larger flow quantity range and a smaller flow quantity range. A flow velocity sensor for the range of the larger quantity of flow is assigned to the range of the larger quantity of flow and a flow velocity sensor for the range of the smaller quantity of flow is assigned to the range of the smaller quantity of flow. The quantity of flow is calculated by switching output signals from the flow velocity sensors.
In the flowmeter constructed by using the plurality of flow velocity sensors as described above, however, the flow of the fluid is disturbed by the existence of one of the flow velocity sensors and there is the possibility that the disturbance exerts an adverse influence on the measurement accuracy of the other flow velocity sensor. As a result, it is difficult to measure the quantity of flow with high accuracy over a wide measurement range of the quantity of flow.
Meanwhile, a gas meter for home use having not only the function of measuring the quantity of flow of passing gas but also a safety function realized by mounting a microcomputer is practically used. According to the safety function, for example, when the quantity of flow of gas equal to or higher than a predetermined quantity is detected or when a predetermined gas flow quantity is detected for predetermined time or longer, a gas emergency trip valve is driven to shut off the gas flow path. By the functions, a leakage in the pipe, unnatural gas outflow, and the like are detected, so that an accident can be prevented and the safety can be assured. For the accurate operation of the functions, however, it is desired to accurately measure the gas flow quantity over a wide flow quantity range.
2. Disclosure of Invention
The invention has been achieved in consideration of the above problems and it is an object of the invention to provide a flowmeter capable of accurately measuring a quantity of flow of a fluid such as gas over a wide flow quantity range.
According to the invention, there is provided a flowmeter comprising: a pipe including a flow path through which a fluid passes and in which a measurement zone for smaller quantity of flow and a measurement zone for larger quantity of flow are provided along the longitudinal direction of the flow path; a flow path dividing member provided in the measurement zone for smaller quantity of flow in the flow path of the pipe to divide the flow path into a plurality of narrower flow paths each having a smaller cross-sectional area; a first flow velocity sensor provided in the measurement zone for larger quantity of flow in the flow path of the pipe, for outputting a signal according to the flow velocity of the fluid passing through the measurement zone for larger quantity of flow; a second flow velocity sensor provided in the narrower flow path formed by the flow path dividing member, for outputting a signal according to the flow velocity of the fluid passing through the narrower flow path; and flow quantity calculating means for calculating the quantity of flow on the basis of at least one of the output signal of the first flow velocity sensor and the output signal of the second flow velocity sensor, in accordance with the quantity of flow.
According to the flowmeter of the invention, a signal responsive to the flow velocity of the fluid passing through the zone is outputted from the first flow velocity sensor in the measurement zone for larger quantity of flow. On the other hand, a signal responsive to the flow velocity of the fluid passing through the narrower flow path formed by the flow path dividing member is outputted from the second flow velocity sensor in the measurement zone for smaller quantity of flow. The flow quantity calculating means calculates the quantity of flow on the basis of at least one of the output signal of the first flow velocity sensor and the output signal of the second flow velocity sensor in accordance with the quantity of flow. In the flowmeter, the scale of the flow velocity distribution in the cross section of each narrower flow path (variations in the flow velocity according to places), that is, the degree of a drift is smaller than that of the flow velocity distribution in the cross section of the whole flow path when the flow path dividing means is not disposed. The flow velocity in the narrower flow path near the flow path wall becomes higher as compared with the case where no flow path dividing means is provided.
According to the flowmeter of the invention, the first flow velocity sensor may be attachable to and detachable from the wall face of the pipe. The second flow velocity sensor may be also attachable to and detachable from the wall face of the pipe.
According to the flowmeter of the invention, the second flow velocity sensor may be disposed in the narrower flow path which is the closest to the wall face of the pipe among the plurality of narrower flow paths. The first flow velocity sensor may be disposed near the wall face of the pipe.
In the flowmeter of the invention, the plurality of first flow velocity sensors may further comprise mean flow velocity calculating means for the measurement zone for larger quantity of flow, which calculates a mean value of the flow velocities in the measurement zone for larger quantity of flow on the basis of the output signals of the plurality of first flow velocity sensors and outputs the mean value to the flow quantity calculating means.
In the flowmeter of the invention, the plurality of second flow velocity sensors may further comprise mean flow velocity calculating means for the measurement zone for smaller quantity of flow, which calculates a mean value of the flow velocities in the measurement zone for smaller quantity of flow on the basis of the output signals of the plurality of second flow velocity sensors and outputs the mean value to the flow quantity calculating means.
The flowmeter of the invention may still further comprise a mesh-like flow regulating member in the flow path.
Another flowmeter of the invention further comprises flow velocity increasing means for increasing the flow velocity of the fluid passing through the narrower flow path in which the second flow velocity sensor is provided.
In the flowmeter, the flow velocity increasing means increases the flow velocity of the fluid passing through the narrower flow path in which the second flow velocity sensor is provided. The flow velocity increasing means can be constructed so as to increase the flow velocity of the fluid passing through the narrower flow path by decreasing the space capacity around the second flow velocity sensor in the narrower flow path. The flow velocity increasing means can be constructed by a pair of column-shaped members which are provided upright on both sides of the second flow velocity sensor. The pair of column-shaped members as the flow velocity increasing means may be provided upright on both sides of the second flow velocity sensor in such a manner that the interval between the pair of column-shaped members is widened toward the upstream of the flow path. The second flow velocity sensor may be integrated into a sensor unit with the pair of column-shaped members as the flow velocity increasing means and the sensor unit may be attachable to and detachable from the wall face of the pipe. At least a part of the pair of column-shaped members as the flow velocity increasing means has a streamline shape along the direction of the flow of the fluid, or the pair of column-shaped members as the flow velocity increasing means may be upright columns each having a cross section of a wing shape.
According to the flowmeter, the plurality of first flow velocity sensors may further comprise mean flow velocity calculating means for the measurement zone for larger quantity of flow, which calculates a mean value of the flow velocities in the measurement zone for larger quantity of flow on the basis of the output signals of the plurality of first flow velocity sensors and outputs the mean value to the flow quantity calculating means. The plurality of second flow velocity sensors may further comprise mean flow velocity calculating means for the measurement zone for smaller quantity of flow, which calculates a mean value of the flow velocities in the measurement zone for smaller quantity of flow on the basis of the output signals of the plurality of second flow velocity sensors and outputs the mean value to the flow quantity calculating means.
According to the flowmeter, the second flow velocity sensor may be disposed in the narrower flow path which is closest to the wall face of the pipe among the plurality of narrower flow paths. The first flow velocity sensor may be disposed near the wall face of the pipe.
The flowmeter may further comprise a mesh-like flow regulating member in the flow path.
According to the invention, there is provided a further another flowmeter comprising: a plurality of flow velocity sensors provided in a flow path through which a fluid passes, for outputting signals according to the flow velocity of the fluid; and flow quantity calculating means for calculating the quantity of flow on the basis of at least one of the output signals of the plurality of flow velocity sensors in accordance with the quantity of flow, wherein each of the plurality of flow velocity sensors is prevented from being influenced by the disturbed flow of the fluid caused by the existence of another flow velocity sensor.
In the flowmeter, since each of the plurality of flow velocity sensors is not influenced by the disturbed flow of the fluid caused by the existence of another flow velocity sensor, a stable output signal can be obtained from each of the flow velocity sensors. The quantity of flow is calculated on the basis of at least one of the output signals, so that stable measurement of the quantity of flow can be realized.
According to the flowmeter, the plurality of flow velocity sensors are disposed out of on a straight line along the direction of the flow of the fluid, thereby eliminating the influence of the disturbed flow of the fluid. In this case, the flow disturbed by the existence of the flow velocity sensor on the upstream side does not reach the flow velocity sensor on the downstream side, so that the output signal from the flow velocity sensor on the downstream side is stabilized.
In the flowmeter, in the case where a flow velocity distribution in a flow path cross section perpendicular to the direction of the flow of the fluid is nonuniform in the direction along the circumferential face of flow path walls forming the flow path, preferably, one of the plurality of flow velocity sensors is disposed in the maximum flow velocity position in the flow velocity distribution in the direction along the circumferential face of the flow path wall. In this case, the flow velocity sensor disposed in the maximum flow velocity position can detect the flow velocity with high sensitivity. xe2x80x9cThe case where a flow velocity distribution in a flow path cross section perpendicular to the direction of the flow of the fluid is nonuniform in the direction along the circumferential face of flow path walls forming the flow pathxe2x80x9d, generally, corresponds to the case where the cross sectional shape of the flow path is not circular.
In the flowmeter, holding units each for holding each of the flow velocity sensors are smoothly embedded in flow path walls forming the flow path without any gap and step, thereby suppressing a disturbance of the flow of the fluid. In this case, since the boundary between the holding units of the flow velocity sensor and the flow path wall is smoothed, when the gas passes there, the flow is not easily disturbed and the influence of the disturbed flow on the other flow velocity sensor is a little.
In the flowmeter, when the positional relation among the plurality of flow velocity sensors is such that one of them is on the upstream side and the other is on the downstream side, it is preferable to further dispose a first mesh-like flow regulating member in the flow path between the flow velocity sensors. According to the flowmeter, the flow of the fluid passed through the flow velocity sensor is regulated by the action of the first mesh-like flow regulating member provided for the flow path between the flow velocity sensors, so that the other fluid sensors are not easily influenced by the disturbed flow.
The flowmeter can be also constructed in such a manner that a part of the plurality of flow velocity sensors is disposed on the upstream side in the flow path, the other flow velocity sensors are disposed on the downstream side in the flow path, and the flow quantity calculating means calculates the quantity of flow in the range of the larger quantity of flow on the basis of output signals of the part of the flow velocity sensors on the upstream side and calculates the quantity of flow in the range of the smaller quantity of flow on the basis of output signals of the other flow velocity sensors on the downstream side. In the flowmeter, the quantity of flow in the range of the larger quantity of flow is calculated on the basis of output signals from the flow velocity sensors disposed upstream and the quantity of flow in the range of the smaller quantity of flow is calculated on the basis of the output signals from the flow velocity sensors disposed on the downstream side. In the range of the smaller quantity of flow, the influence of the disturbed flow caused by the existence of the flow velocity sensors on the upstream side is not easily exerted on the flow velocity sensors on the downstream side. Consequently, an output signal from the flow velocity sensor disposed on the downstream side is not unstable.
According to the flowmeter, a flow path dividing member for dividing the flow path into a plurality of narrower flow paths each having a smaller cross-sectional area may be further disposed in the flow path.
In the flowmeter, a second mesh-like flow regulating member may be further disposed in the flow path on the upstream side of the plurality of flow velocity sensors. In the flowmeter, at least the flow of the fluid passing through the flow velocity sensor on the most upstream side is regulated by the action of the second mesh-like flow regulating member.
In the flowmeter, at least a part of the plurality of flow velocity sensors may be disposed near the wall face of the flow path.
Other and further objects, features and advantages of the invention will appear more fully from the following description.